Performance of a foil or surface in a flow of fluid such as air or water is critical for a system performance, affecting lift, drag and vibration of a system.
The leading section of the foil is usually an area of increasing thickness and results in a thin laminar boundary layer until such point that viscous drag, surface friction or pertuberances causes turbulence to occur in the boundary layer. The turbulent boundary layer has characteristically higher drag than the laminar flow region, however may also have improved stability of flow. The development of an adverse pressure gradient results in separation of the flow from the surface, and a further large increase in drag occurs from this point rearwards. While a foil section may be designed to maintain a large area of laminar boundary layer, practical limitations of manufacture and cleanliness generally preclude widescale laminar boundary layer development.
Noise signature of a blade, or other foil is affected by the vortex development in the wake of the section. Additionally, lift and drag performance can be affected greatly by the use of trailing edge modifiers. In practice, this performance is not attained due to constraints of engineering a suitable mechanism.
Micro Vortex generators, microVG's, are fabricated from a rigid material such as aluminium are used to reenergise boundary layers. Large Eddy Breakup Units, or LEBU's are occasionally used to adjust a boundary layer condition, and are constructed from rigid materials. A drag modifying surface is manufactured by 3M under the tradename “Riblet”. This surface is a thin textured film, designed to provide a reenergising of the boundary layer to reduce surface drag. Alternatively, a rigid surface may be deformed by fluting or indentations that act as a form of flow modifier.
To change acoustic signature and/or lift/drag performance, fluting of the trailing edge of a foil or section has been accomplished, and tabs such as lift enhancing tabs or gurney tabs have been applied in experimentation. Fluting has been accomplished on jet engine exhaust systems in current art.